Conventionally, methods for sterilizing or completely sterilizing various microorganisms such as bacteria or viruses can be broadly divided into two types, i.e., a physical method (mechanical method) using heat, pressure, or the like, and a chemical method using chemical agents.
According to the physical method, the type of an object to be sterilized is limited because the object to be sterilized is exposed to extreme physical conditions in many cases. In the case of the chemical method, chemical agents to be used may exert a harmful influence on the human body. Therefore, the chemical method involves a process for securely rendering the residual agent harmless, which consequently requires more cost and time.
The inventors of the present invention previously proposed the sterilization method using low-temperature plasma generated under an atmospheric pressure (WO2009/041049; “Strong bactericidal activity of the plasma treated water for medical application based on the reduced pH method” by Katsuhisa Kitano, Satoshi Ikawa, Atsushi Tani, Yoichi Nakashima, and Tomoko Ohshima on February 2014, 8th International Conference on Reactive Plasmas/31st Symposium on Plasma Processing, http://plasma.ed.kyushu-u.ac.jp/icrp-8/; “Sequential production of high concentration plasma treated water for disinfection” by Katsuhisa Kitano, Satoshi Ikawa, Yoichi Nakashima, and Atsushi Tani, on March 2014, The Japan Society of Applied Physics, Spring Lecture Presentation, https://confit.atlas.jp/guide/event/jsap2014s/top”; and “Cryopreservation of plasma treated water (PTW) for disinfection” by Katsuhisa Kitano, Satoshi Ikawa, Yoichi Nakashima, and Atsushi Tani on May 2014, 5th International Conference on Plasma Medicine (ICPM5), http://icpm5.plasmabio.com/). The sterilization method is a method for sterilizing microorganisms present in a liquid or on a surface thereof by generating plasma in a vicinity of or in a manner to make contact with a liquid whose pH value is adjusted to become 4.8 or lower and allowing radicals generated by the plasma to make contact with the liquid. According to the sterilization method, superoxide anion radicals (O2−.) that are generated by the plasma are diffused into the liquid, and the diffused superoxide anion radicals (O2−.) react with protons (H+) in the liquid to thereby form hydroperoxy radicals (HOO.). The concentration of hydroperoxy radicals (HOO.) is increased by adjusting a pH value of the liquid to become 4.8 or lower, so that powerful microbiocidal activity is attained.
Another sterilization method has been proposed in which plasma is generated in a manner not to contact a liquid, active species generated by the plasma are electrophoresed to contact with the liquid, and thereby, the liquid is sterilized (WO2011/027542).
The plasma generation device used for the sterilization methods is disclosed in WO2008/072390.
However, in order to use plasma for odontotherapy or for sterilization and disinfection of medical equipment, for example, a plasma generation device should be installed in a place where medical treatment is provided, and further, a pipe line for introducing different types of gases should be provided. This causes problems related to cost and space.
In order to solve the problems, the inventors of the present invention proposed a sterilization method. The method includes using plasma to produce a plasma-treated solution in which biocidal activity is held; freezing the plasma-treated solution to produce solid ice and store the solid ice in a frozen state; carrying the solid ice to a place where medical treatment is provided; thawing, in the place, the solid ice to return to the plasma-treated solution in which biocidal activity is held; and using the plasma-treated solution to apply a sterilization treatment (WO2013/161327).
As discussed above, the use of plasma or the use of a plasma-treated solution produced by physical reaction by the plasma achieves powerful microbicidal activity. It is known that the microbicidal activity of the plasma-treated solution comes from the hydroperoxy radicals (HOO.). However, it was not revealed what supplies the hydroperoxy radicals (HOO.). In other words, it was previously unknown what kind of substance is a precursor of the hydroperoxy radicals (HOO.).
If the precursor of the hydroperoxy radicals (HOO.) were identified, and further, if the substance identified were synthesized by chemical reaction, hydroperoxy radicals (HOO.) having higher concentration could be supplied, leading to the possibility of achieving more powerful microbicidal activity.
The inventors of the present invention have continued to conduct further researches and experiments. The inventors then successfully identified what kind of substance supplies the hydroperoxy radicals (HOO.) to produce powerful microbicidal activity. Further, the inventors obtained knowledge about a method for efficiently synthesizing the substance by chemical reaction, and about conditions necessary to perform powerful sterilization using the synthesized substance.